1. Field of the Invention
The present invention relates to a hybrid vehicle. More specifically, the present invention relates to a hybrid vehicle which uses an engine and a motor capable of generating power as a source of power, and includes a torque converter provided with a lock-up clutch and a transmission between the source of power and an output shaft.
2. Description of Related Art
Recently, hybrid vehicles including an engine and a motor capable of generating power as a source of power have been developed to conserve fuel for driving the engine and to decrease exhausted gases generated by combusting the fuel.
In this type of hybrid vehicle, power input from vehicle's wheels during deceleration of the vehicle is generally transmitted to the motor, and a regenerative operation is performed by the motor to convert the deceleration energy into regenerative energy so that the energy may be stored in a battery as electrical energy. Hereinafter, the regenerative operation carried out by the motor during the deceleration of the vehicle is called regenerative braking.
Also, as described in Japanese Unexamined Patent Application, First Publication No. Hei 9-9415, for example, some of the above-mentioned type of hybrid vehicle include a torque converter and a multistage automated transmission between the source of power and driving wheels.
In such hybrid vehicles, a gear change point at which the gear of the multistage automated transmission is determined, is shifted to a direction increasing the high speed gear region during the regenerative braking so that the revolution rate of the engine is set to be smaller and the energy consumed by the engine brake decreases, thereby enhancing the regenerative efficiency by increasing the regenerative energy to increase the regenerative electric power generated.
Moreover, in the above-mentioned hybrid vehicle, the lock-up clutch is disengaged during the regenerative braking so that the resistance of the engine decreases and the regenerative power further increases.
However, there are problems for a hybrid vehicle having a structure in which the input side of the motor is connected to the output side of the engine, the input side of the torque converter provided with the lock-up clutch is connected to the output side of the motor, and the driving wheels are connected to the output side of the torque converter, that the regenerative efficiency decreases and the regenerative power is reduced if the regenerative braking is carried out when the lock-up clutch is disengaged since the torque (i.e., regenerative torque) at which the regenerative operation is carried out by the motor depends on, or is restricted by, the capacity of the transmitted torque from the torque converter.